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MAGNESIA (MAGNESIUM OXIDE)                   71

(i) The other is rendered alkaline with ammonia, treated with excess
of silver nitrate solution, shaken and filtered. The filtrate is then acidified
with nitric acid, a precipitate being formed in presence of chlorine or bromine
but only an opalescence, and that not immediate, when the iodine is pure.

4. Quantitative Determination of the Iodine.In absence of appre-
ciable proportions of chlorine or bromine, it is sufficient to dissolve a given
weight (0-1-0-2 gram) of the iodine in potassium iodide solution (i : 10)
and to titrate with thiosulphate solution in presence of starch paste (i c.c.
of N/io-thiosulphate = 0-0127 gram of iodine).

In presence of chlorine and bromine, the iodine is dissolved in sodium
hydroxide solution, sodium bisulphite solution and ferric chloride being
added and the solution acidified with hydrochloric acid ; the liquid is then
distilled in a suitable apparatus until all the iodine passes over. The iodine
is collected in potassium iodide solution and titrated with thiosulphate.1

Crude commercial iodine may contain as much as 22% of moisture,
and usually contains 74-94% of iodine. Restiblirned iodine should contain


The pure salt is in transparent, colourless crystals, which in dry air
effloresce, losing water and acetic acid and absorbing carbon dioxide. The
crude salt (pyrolignite of lead) forms yellowish fused masses of empyreumatic

The pure acetate should answer to the tests 1-4; in the crude acetate
the content of acetic acid is determined as in 5.

1.  Solubility.i part in 5 parts of distilled water should give a clear,
colourless solution.

2.  Chlorides,   Sulphates.The  i : 20 solution  should not  become
turbid with either silver nitrate or barium chloride, even on standing.

3.  Copper.The   i : 10  solution,   treated  with  excess  of  ammonia
and filtered, should give a colourless liquid.

4.  Iron, Alkaline Earth and Alkali Metals.The 1:20 solution
treated with dilute sulphuric acid (or acidified with dilute hydrochloric
acid and treated with hydrogen sulphide) until the lead is completely pre-
cipitated, should give a filtrate which leaves no appreciable residue on
evaporation and ignition.

5.  Determination of the Acetic Acid.This is carried out as in
calcium acetate  (q.v.}.    i c.c. N-alkali = 0-379 gram of Pb(CaH302)a +

MAGNESIA (Magnesium Oxide)

MgO = 40-36 (40)

Pure magnesium oxide (Magnesia calcinata) is a light, white, amorphous
powder which may contain, as impurities, magnesium carbonate and small

1 Topf's method, Zeitschr. f. analyl. Chem., 1887, p. 288.hiosnlphuto to